1. Field of the Invention
The invention relates to a data transmission system using a transmission code in which one of the data states corresponds to a zero transmission level, the system including an adaptive signal processing circuit and means for detecting the data state represented by the received transmission code at fixed intervals.
2. Description of the Related Art
Digital data transmission systems for use in the local telephone network include signal processing circuits which are necessarily adaptive in order to cope with the wide range of line parameters encountered. Examples of such circuits are echo cancellers and feedback equalisers.
There are a number of different adaptation algorithms known at present. The simplest is to use special start-up sequences but this method is not often followed because of complications with the overall protocol and, in addition, the system cannot track changes which occur after initialisation. Instead, a number of known systems use some form of Least-Mean-Square algorithm whereby the coefficients are adjusted according to the gradient of the squared-error. Of course, once the residual echo becomes smaller than the received signal, then the error is not directly available. However, the correlation process used to determine the gradient of the square error also serves to extract the error from the total signal. This form of updating is termed the Zero Reference algorithm and is fully described by N. A. M. Verhoeckx, H. C. v. d. Elzen, W. A. M. Snijders and P. J. v. Gerwen in a paper entitled "Digital Echo Cancellation for Baseband Data Transmission" published in IEEE Transactions on Acoustics, Speech and Signal Processing, Vol. ASSP-27, No.6, December 1979, at pages 768-781. The disadvantage of this algorithm is the presence of the received signal. From the point of view of the echo-canceller, the received signal represents a large noise-like element which can only be removed by averaging over a large number of samples. This averaging leads to a long convergence time. In addition, the word-length of the coefficients has to be increased to accommodate the averaging process. Further, if a one-bit A/D converter is used then a dither signal, equal in amplitude to the received signal, has to be used as a reference for the converter to enable the error to be encoded.
To overcome this disadvantage, the Adaptive Reference algorithm has been developed. With this technique, which is described in a paper by D. D. Falconer entitled "Adaptive Reference Echo Canceller" published in IEEE Transactions on Communications", Vol. COM-30, No. 9, September 1982 at pages 2083-2094, a separate adaptive mechanism is used which learns to simulate the received signal. This is then subtracted from the total signal at the input to the echo-canceller leaving only the error signal to control the canceller. The disadvantage of this method is the further complication of the adaptive reference system.